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The Infamous R: What is the Reproduction Number?

You may have seen graphs or media reports referring to R, the reproduction number; however, if you are like most people, you probably still have a few questions. In this blog post we’ll try to explain this concept and how it is relevant to the current pandemic. At the most basic level, R is the number of new cases of an infectious disease generated by a single case. That is to say, it is the average number of people that one infected person would be expected to go on to subsequently infect.

The actual reproduction number is constantly changing over time and is affected by many things, including the intrinsic contagiousness of the virus, the level of population immunity, the season, current mitigation measures and viral evolution. When R is below 1, infections cannot replace themselves and the epidemic extinguishes itself. Imagine if every couple(2 people) in a given country had only 1 child – this population is eventually going extinct. Conversely, when R is above 1, exponential growth of infections is seen. Earlier in the pandemic you probably heard the words “flatten the curve”, and what this actually meant was getting R as low as possible. Let’s go over each of variables that affect R at any given time:

  • Intrinsic contagiousness. Some viruses are very contagious and others are much less so. This measure of intrinsic contagiousness is known as R-zero. For example, the R-zero of measles is estimated to be about 15, which is the highest of any disease known to infect humans. Put otherwise, in a population with no immunity, any given person with measles would be expected to infect, on average, 15 other people. The R-zero of the ancestral strain of SARS-COV-2 was about 3, thankfully much lower than measles.
  • Population immunity. A higher proportion of immune individuals will push down the reproduction number, as there are fewer chances any given person can fall ill. To continue with our above example, most people in North America are vaccinated for measles – therefore immune – and the actual reproduction number is not anywhere near 15 during an outbreak. As SARS-COV-2 was a never-before-seen virus in 2019, population immunity was nonexistent.
  • Seasonality. Many viruses transmit better during winter months. This is due to many factors including cooler temperatures, dryer air and more time spent indoors. Summer weather, on the other hand, will push down the reproduction number.
  • Mitigation measures. Medical masks and respirators, hand hygiene and physical distancing are effective in reducing the spread of Covid-19 and push down the reproduction number. Business & school closures and restrictions on gatherings also push down the reproduction number by reducing the number of contacts each person has, but at much higher societal cost.
  • Viral evolution can increase the reproduction number, and this can happen when a virus evolves the ability to evade the immune response or becomes better at multiplying and infecting humans. Viral evolution can drastically change the course of a pandemic as we have seen recently with the highly-transmissible Omicron variant of SARS-COV-2.

Although the basics of the reproduction number are simple, the many factors at play are complex. One thing is for sure: we’re all in this together, so let’s all continue to do our part to keep R down. That means following public health guidance with respect to vaccination, testing, isolation and masking.

Until next time, stay safe and mask up!


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